The invention relates to a multilayer adhesive tape having a backing based on an oriented thermoplastic film coated on one side with adhesive composition. The adhesive tape indicates the unauthorized opening of a package by virtue of the fact that, on peeling from the surface to which it is stuck, areas of the adhesive tape are extracted from the adhesive tape and remain on the surface as a message, which is imperceptible prior to detachment from the surface. The message remains visible even if following detachment the adhesive tape is replaced exactly at the original bond site.
Protecting goods present in the logistical chain from producer to user has become more difficult owing to the large number of links in this chain. Thus the damage which occurs by theft of the product from its delivery packaging during warehousing and transit has risen to become a significant cost factor. The recipient of a delivery unit in which some of the product has been switched or removed by theft, moreover, has difficulties in proving that this act has not been committed within his or her own sphere of influence.
Significantly greater security against an attempted theft, and reliable evidence that a delivery unit has been opened before its transfer to the next section of the logistics chain, are made possible by an adhesive tape which indicates by means of an irreversible message that it has already been detached from the surface of the delivery unit.
Security systems for detecting unauthorized opening are known and are described in multiplicity in the prior art. Security labels, in particular, are of great importance in this context. For example, U.S. Pat. No. 4,184,701 describes security labels which are constructed from a plurality of color layers or, generally, layers and which as a result exhibit little adhesion of the product layers to one another. In combination with a strongly adhering adhesive composition, such layers separate from one another irreversibly, thereby indicating the broaching of the package.
Furthermore, for example, DE 34 31 239 A1 describes the use of security labels comprising a backing material of low tear propagation resistance. In combination with a strongly adhering adhesive composition, the security label is irreversibly destroyed on removal and thus indicates the attempted broaching.
The security label has a lower, first adhesive layer, intended for detachment from a backing and application to the package, on the bottom face of a first label material to whose top face there is applied, by means of a second adhesive layer, a second label material. The extent of the bond strength of the first adhesive layer between the package and the first label material is different than the extent of the bond strength of the second adhesive layer between the first label material and the second label material.
Adhesive security tapes are commonly designed in such a way that a weakness in anchoring below the colored structure of the adhesive tape is introduced in the region of the writing. For instance, U.S. Pat. No. 5,633,058 describes a transparent printed indicium, anchored weakly to the backing film, which is covered with a full-area colored layer which anchors well to the backing film and to the first print. The colored layer is covered with a self-adhesive composition. When an adhesive security tape of this kind is peeled from a surface, the transparent printed indicium is parted from the film surface, together with the color layer and the adhesive composition, and remains as writing on the surface.
EP 0 491 099 A1 describes a printed indicium comprising a silicone material likewise covered with a colored layer which adheres well to the film. The colored layer is covered with a self-adhesive composition. When an adhesive security tape of this kind is peeled from a surface, the color layer and the adhesive composition detach from the silicone surface and remain as writing on the surface.
The disadvantage of a construction of this kind is the complex process engineering for producing the layers, since partial silicone surfaces greatly hinder further coating, owing to the low surface tension.
U.S. Pat. No. 4,121,003 describes how the transfer of writing is brought about by this writing comprising of a material of low cohesion which, when the adhesive security tape is detached, splits within itself and remains in part on the surface to which it was stuck.
EP 0 404 402 A2 is based on a transparent splittable layer which becomes opaque at the boundary layer which develops, and which therefore offers a previously imperceptible optical contrast surface against which the writing then becomes legible.
A disadvantage of structures of this kind, moreover, is that the surface of delivery units which comprise not very split-resistant material such as paper, for example, are easily torn by the adhesive tape. The split-off surface of the delivery unit tears over its full area beneath the adhesive tape and adheres in such a way that it covers the surface of the adhesive composition. In that case, the writing is not transferred to the surface of the cardboard box, and the message provided does not appear.
This disadvantageous circumstance is addressed by many available adhesive tapes of this kind by virtue of the fact that they possess a weakly adhering adhesive composition which may be peeled off without tearing the paper surface. The disadvantage of this solution, however, is inadequate bonding reliability to the different kinds of paper materials, especially grades with a high waste paper content, so that the U-shaped closure of a delivery case carrying these adhesive tapes is parted again under the pressure of the lid flaps, and the case opens of itself.
A further requirement of security against manipulation is the rapidly and easily visible individuality of the adhesive tape closure. Normal adhesive packaging tapes, therefore, are printed on the reverse of the backing with the individual company logo, so that a switch or overtaping with a different adhesive tape is readily perceptible. The possible print run of adhesive tapes identified in this way is a single roll at minimum, so that a high level of individuality of the delivery of the packages sealed using this adhesive tape may be achieved.
To date there has been no satisfactory success in combining the two security effects of message transfer and individual printing. Subsequent printing on the reverse of the backing of the existing adhesive tapes is impossible using the standard printing techniques, owing to the necessary silicone release on this surface. In the prior art embodiments of such an adhesive tape, therefore, the backing film is printed prior to the provision of the layers necessary for the message transfer effect. A consequence of this is that only large runs of adhesive tapes with an individual print can be produced economically.
Available embodiments of an adhesive security tape in the sense described are printed, as an alternative, with a serial number for the purpose of individualization. This method, however, requires a high level of information exchange within the logistics chain. In addition, it necessitates laborious checking of each delivery unit for agreement with the circle of numbers notified by the producer.
The technical implementation of an adhesive security tape of this kind is very costly in the material and in the production process. For instance, backing films used to date have been those comprising expensive materials such as polyesters and polystyrene. Films based on polystyrene, however, are brittle and do not withstand the impact and drop loads which occur during transport and warehousing of packages sealed with adhesive packaging tapes.
Thermoplastic films based on polyethylene terephthalate (PET) or polyvinyl chloride (PVC) are used by various manufacturers to produce adhesive tapes. Films based on PET, in particular, are notable for a high elongation at break and heat resistance of from 130° C. to 175° C., in addition to resistance to dilute alkalis and acids.
Furthermore, films based on polyesters possess very high abrasion resistance and puncture resistance, but are less widespread in the adhesive packaging tapes sector owing to their relatively high price relative to polyolefin-based films.
Films based on PVC are used, in combination with solventborne adhesive compositions based on natural rubber, by various manufacturers as adhesive packaging tapes. In this case, unplasticized PVC films are used owing to their better heat resistance, which extends up to max. 105° C.
The PVC in this case may be obtained by emulsion, suspension or bulk polymerization. Copolymers based on PVC/vinyl acetate are also known.
Furthermore, the films based on unplasticized PVC are notable for moderate puncture strength and partial resistance to gasoline, oil and alcohols. In general, in the absence of corona treatment, the anchoring of primers to PVC films is better than in the case of other thermoplastic films, such as polyolefin-based films, for example. This is a result of the high surface energy even without pretreatment [approximately 39 dyn/cm] and on the rough surface texture of PVC films. For adequate anchoring of adhesive compositions based on natural rubber, a primer-coated PVC film is advisable.
In the production of adhesive tapes with adhesive compositions based on natural rubber, both water-based and solventborne primers are used as adhesion promoters between adhesive composition and backing film. These adhesion promoters which are used possess in part a crosslinking effect on the adhesive composition based on natural rubber which is applied from solution.